Such a method is implemented, in particular, when manufacturing optical fibers that are to form the basic elements of a telecommunications cable.
In a known manner, the optical fiber is manufactured by being drawn from a preform. While drawing is taking place, the fiber is coated in one or more coatings which give it mechanical strength and thus the required optical performance.
In general, the optical fiber is coated in a first or "primary" coating which increases its traction strength and protects it from moisture. A second or "secondary" coating is applied onto the substrate constituted by the fiber plus its primary coating and serves to distribute within the primary coating any compression and bending forces to which the fiber may be subjected.
The ability of fibers to withstand microbending depends mainly on the effectiveness of the secondary coating(s). Sensitivity to microbending is measured by a standardized test corresponding to utilization standards for fibers and for cables containing them.
Improving the ability of an optical fiber to withstand microbending depends both on the thickness and on the Young's modulus of its coating(s), in general obtained from materials which are cured by polymerization produced by ultraviolet radiation. The temperature at which polymerization takes place determines the Young's modulus of such coatings.
In European patent application EP-0 314 174, proposals are made to guarantee Young's modulus lies within a satisfactory range by controlling the temperature of an enclosure in which the coating is polymerized. The fiber on which the material has been deposited passes through the enclosure where it is exposed to a halogen lamp whose spectrum contains ultraviolet rays and infrared rays.
The ultraviolet rays cause the material to polymerize. Under the action of the infrared rays, the temperature inside the enclosure rises and under steady conditions it reaches about 400.degree. C. in the vicinity of the halogen lamp and about 200.degree. C. in the vicinity of the fiber.
Since the mean polymerization temperature of the material used in that patent application, and also of most UV-cured resins is about 100.degree. C., provision is made to cool the enclosure.
To this end, a flow of steam mixed with carbon dioxide is applied to absorb the infrared rays and remove the corresponding energy from the enclosure.
Methods of the kind described above can give rise to drawbacks, in particular by requiring complicated temperature control which makes use of a gas flowing through the enclosure.